In the 970s, in the wars for control of Brittany, he subjugated the county of Rennes and Count Conan I affirmed the rights of his family in the region. Around 977, his father died and he succeeded in his counties.

Near 995, he entered into a war against Fulk, who was already at war with Geoffrey I of Brittany. Odo allied with his brother-in-law William IV of Aquitaine and Baldwin IV of Flanders. Even his old enemy, Richard of Normandy joined in the war on Fulk. In the winter of 995 – 996, they besieged Langeais, however Odo became ill and was taken to the monastery of Marmoutier at Tours where he died on 12 March 996.[2][3]